Objective: To develop reference ranges with gestational age for the pulsatility index in the umbilical artery (UA-PI) and fetal middle cerebral artery (MCA-PI and the cerebroplacental ratio (MCA-PI / UA-PI) and examine the maternal characteristics and medical history that affect these measurements.
Patients and methods: This was a cross-sectional study of 72,417 pregnancies undergoing routine ultrasound examination at 20+0 to 22+6 weeks’ gestation (n=3,712), or at 31+0 to 33+6 weeks (n=29,035) or at 35+0 to 36+6 weeks (n=37,282) or at 41+0 to 41+6 weeks (n=2,388). For the purpose of this study we included data for only one of the second or third trimester visits. The inclusion criteria were singleton pregnancy, dating by fetal crown-rump length at 11+0 to 13+6 weeks’ gestation, livebirth of morphologically normal neonate and ultrasonographic measurements by sonographers that had received the Fetal Medicine Foundation Certificate of competence in Doppler ultrasound. Since the objectives of the study were to establish reference ranges, rather than normal ranges, and to examine factors from maternal characteristics and medical history that affect these measurements, we included all pregnancies having routine ultrasound examinations irrespective of whether the mothers had a pre-existing medical condition, such as diabetes mellitus, or a pregnancy complication, such as preeclampsia or suspected fetal growth restriction. Median and standard deviation (SD) models were fitted between UA-PI, MCA-PI and CPR and gestational age. Assessment of goodness of fit of the models was by inspection of quantile to quantile (q-q) plots of z-scores calculated via the mean and SD models. The distributions of MCA PI, UA PI and CPR z-scores were examined in relation to maternal characteristics and medical history.
Results: The relationship between the median and gestation age was linear for UA-PI and cubic for MCA-PI and CPR and the SD was log quadratic for all three. MCA-PI and CPR increased with gestational age from 20 weeks’ gestation to reach a peak at around 32 and 34 weeks’ respectively, and decreased thereafter, whereas UA-PI decreased linearly with gestation from 20 to 42 weeks. Compared to the general population, significant deviations in MoM values of UA-PI, MCA-PI and CPR were observed in subgroups of maternal age, BMI, racial origin, method of conception and parity.
Conclusion: The study established new reference ranges of UA-PI, MCA-PI and CPR with gestational age and reports maternal characteristics and medical history that affect these
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